1. Field of the Invention
The present invention relates to an injection molding apparatus for high-gloss products, and more particularly, to a weldless injection molding apparatus which can rapidly heating and cooling an injection mold during injection molding of a product using an injection mold.
2. Description of the Related Art
Injection molding is a process for forming products by applying heat to polymer resin to be plasticized and then injecting melted polymer resin into a mold using a hydraulic force. According to the injection molding, products of different sizes may be mass-produced.
In general, forming of the polymer resin has demerits, such as low surface glossiness and poor outer appearance due to a weld line generated as the melted polymer resin meets the inside of a mold.
To overcome the demerits, a hot pressing method has been widely used in which a temperature for injection molding is set to be higher than a melting temperature of a polymer resin. Examples of the hot pressing method are disclosed in Japanese Patent Publication No. sho 45-22020 entitled “Method of Heating Using Hot Wind”, Japanese Patent Publication No. sho 51-22759 entitled “Method of Heating Using Electric Heater and Method of Cooling Using Water”, Japanese Patent Publication No. sho 55-109639 entitled “Method of Heating Using High Frequency Induction”, Japanese Patent Publication No. sho 57-165229 entitled “Method of Heating by Blowing Vapor into Cavity”, Japanese Patent Publication No. sho 61-79614 entitled “Method of Inserting Heat Plate between Cavity and Core”, and Japanese Patent Publication No. hei 4-265720 entitled “Method of Heating Surface of Mold by Electric Conductive Layer”.
However, when polymer resin is formed by setting the temperature of injection molding higher than the melting temperature of the polymer resin, no weld line is generated and outer appearance, such as glossiness, may be improved. In contrast, a cooling time extends due to the high mold temperature and an overall forming cycle extends so that production efficiency may be deteriorated. Also, since the polymer resin is disassembled from the mold by being cooled down under the melting temperature, deformation due to contraction becomes severe compared to a general injection case.
The mold heating methods, which have been used at initial stages, require quite a long time to heat a mold surface because of their low heating rate. Accordingly, an overall cycle time for forming molded products may extend, resulting in poor production efficiency. To address the problem associated with low heating rate, the temperature of a mold may be set to be relatively high. In such a case, however, a cooling time may be prolonged.
Considering the above issues, a system for automatically controlling temperature of an injection mold was suggested and disclosed in Korean Patent Registration No. 811909. In addition, Korean Patent Registration No. 167711 discloses a mold cooling system; Korean Patent Registration No. 470835 discloses a mold temperature control system; and Korean Patent Registration No. 701229 discloses a structure in which rapid heating and cooling of a mold is carried out within a range between 100° C.-200° C.
With the above structures, however, the issues stated above cannot be overcome because the heating and cooling of an injection mold is not carried out within a short time. In particular, the injection structure requires a lot of cooling lines and hot water supply lines for heating and cooling so that the structure of a mold may be relatively weak and thus the mold may be deformed during a repeated number of forming cycles.